Project Summary In the normal brain, glial cells grow and differentiate under strict controls that match the mature glial population to the local population of neuronal partners. Defects in the mechanisms controlling glial cell growth contribute to oncogenesis in the brain and peripheral nervous system. Conversely, insufficient glial cell numbers are associated with functional deficits in multiple neurological diseases. However, signals coordinating glial cell growth and differentiation are not understood. Here, we address cues that coordinate the proliferation and pro- myelinating differentiation of Schwann cells. Schwann cells in myelinated peripheral nerves establish 1:1 relationships with axons. We found peripheral nerves in mice lacking a pair of matrix proteins, laminins-2 and - 8, are fully "amyelinated". Specifically, Schwann cells lacking these proteins on their surfaces fail to proliferate to match the number of axons, and fail to isolate and myelinate individual axons. Laminins-2 and -8 are the principal glycoproteins in the Schwann cell basal lamina. Contrary to previous assumptions, however, we found they do not act by promoting basal lamina formation. Rather, laminin-2 and -8 each contributes a critical signaling activity, which together promote Schwann cell proliferation and pro-myelinating differentiation. Based on further evidence, we hypothesize laminin-2 and laminin-8 differentially regulate proliferation and differentiation through separate receptor pathways. This project will test this hypothesis by combining loss-of- function mutations in primary laminin receptors with loss-of-function mutations in laminin-2 and laminin-8. We predict distinct defects in the ability of Schwann cells to proliferate and/or differentiate predicted for specific mutant combinations. Quantitative and immunochemical methods will be used to characterize Schwann cell development. By identifying signaling pathways that choreograph neuron:glia interactions in developing nerves, the results will guide the development of therapeutic targets to improve recovery following nerve injury, slow the progression of neurological diseases, and arrest neural cancers. PUBLIC HEALTH RELEVANCE: The number of glial cells in myelinated nerves precisely matches the size and number of neuronal processes. Defects in controlling glial cell growth and differentiation cause brain cancers, and inhibit recovery of neural function following neural injuries and demyelinating diseases. However, the mechanisms controlling glial cells in normal development are not understood. This project will study how the growth of peripheral nerve glial cells, called Schwann cells, is regulated by dominant signaling components concentrated in the extracellular matrix of the developing nerve.